The mammalian primary visual cortex is a model system for understanding cortical integration and processing of information. This network consists of an elaborate system of intrinsic connections within and between the six cortical layers. In the first few months of life, the complexity of this circuitry increases tremendously, paralleling the emergence of distinct visual response properties in each layer. The primary goals of the proposed research are to define the roles of activity-independent factors, spontaneous neuronal activity, and patterned visual experience in structuring and modifying horizontal and vertical connections. To determine the mechanisms generating specific vertical connections, the modifiability of the laminar specificity of layer 2/3 pyramidal neurons will be examined. The effects of blocking either patterned visual experience, by binocular lid suture, or of spontaneous neural activity, by intraocular tetrodotoxin injections, will be assessed with single-cell labelling techniques in brain slices. Reconstructions of individual axon arbors will determine whether the normal laminar specificity, in which collaterals are absent from layer 4, breaks down after manipulations of activity levels. To further characterize the signals involved in generating laminar specificity, slices of immature cortex will be maintained in tissue culture in order to differentiate between activity-dependent and activity-independent mechanisms. Manipulations of visual experience will also be used to examine the mechanisms underlying the emergence and refinement of clustered horizontal connections in layer 2/3. Binocular tetrodotoxin injections, and in vitro cultures of tangential brain slices, will reveal whether the emergence of the initial crude pattern of horizontal connections requires spontaneous neural activity. Varying periods of binocular deprivation, followed by restoration of normal vision, will be used to assess the "critical period" during which these intrinsic horizontal connections can be modified by visual experience. Animals will also be reared with induced strabismus to parse out the effects of the amount of visual experience versus the pattern of visual experience on the refinement of clustered connections. While previous investigators have emphasized the relationships between ocular dominance columns and visual activity, the experiments proposed here will provide the first analysis of the involvement of activity-dependent and activity-independent mechanisms in the differentiation of visual cortical circuits. The conditions of cataract-induced binocular deprivation and strabismus cause profound visual dysfunction in humans, and affect a considerable number of young children. The proposed experiments will provide insight into the consequences of these deficits on local circuits in the striate cortex. They will furthermore help determine when intervention can rescue deprivation-induced deficits. Understanding the basic mechanisms should also provide a rationale for new therapies for treating or preventing the cortical consequences of abnormal visual input. Finally, uncovering activity-independent cues involved in circuitry formation may lead to the discovery of novel factors critical to normal brain differentiation.